“The Forrester Wave™: Global Managed Security Services Providers, Q3 2020” and “The Forrester Wave™: Managed Detection And Response, Q1 2021” confirmed that significant overlap exists between managed security services (MSS) and managed detection and response (MDR) services. Yet another approach, security operations center as a service (SOCaaS), has materialized, further complicating decisions for security leaders. Our evaluations show that these vendors and their potential customers share the confusion. This report provides standard classifications, similarities, differences, and use cases for security leaders to differentiate among MSS, MDR, and SOCaaS vendors.